1. Field of the Invention
The invention relates to an image processing apparatus and method for processing image data and, more particularly, to an image processing apparatus and method for obtaining an image of a good picture quality.
2. Related Background Art
Hitherto, according to the standard of a G3 facsimile apparatus, a scanning line density for reading or recording is predetermined to 8 dots/mm for the main scanning direction and to 3.85 lines/mm (standard), 7.7 lines/mm (fine), and 15.4 lines/mm (super fine) for the sub scanning direction.
In almost of the conventional facsimile apparatuses, therefore, both of the reading means and the recording means use a system corresponding to the above scanning line densities. Particularly, with respect to the recording means, in many cases, a thermal recording system is used from viewpoints of easiness of the control and a reduction of a space.
However, in case of the thermal recording paper, since it cannot be preserved because of discoloration and the after-writing (namely, characters or the like are written to the paper after the recording) cannot be performed, a facsimile apparatus using a normal paper as a recording paper is rapidly widespread.
As a normal paper recording system in this case, an LBP (laser beam printer) system or an ink jet system is generally used. However, in such LBP or ink jet recording apparatus, ordinarily, a resolution is set to about 400 dpi and is very higher than a scanning line density of the G3 standard. In case of performing the G3 reception and recording or in case of copying and recording a read original, therefore, a resolution of image data has to be converted in accordance with a resolution of the recording apparatus and, after that, the image data is recorded. A specific example of such a case will now be described hereinbelow.
FIG. 20 shows a block diagram of a conventional normal paper recording facsimile apparatus.
In FIG. 20, reference numeral 2001 denotes a contact sensor unit (CS) to read an original image. In the conventional apparatus, the original image is read at a resolution of 8 dots/mm in the main scanning direction and, for example, 7.7 lines/mm in the sub scanning direction.
Reference numeral 2002 denotes an analog image processing unit for executing analog image processes such as DC reproduction, shading correction, and the like to an analog output of the CS 2001; and 2003 indicates an A/D conversion unit for converting an output of the analog image processing unit 2002 to digital data of multivalues every pixel.
Reference numeral 2004 denotes a luminance/density conversion table unit for LOG converting luminance data of each pixel as an output of the A/D conversion unit 2003 to density data; and 2005 indicates an error diffusion processing unit for executing a half-tone process to multivalue output data of the luminance/density conversion table unit 2004 by an error diffusing method, thereby producing binarized image data.
In the above error diffusing process, a binarizing process of a target pixel is executed by using an error diffusion matrix shown in FIG. 21. As is well known hitherto, according to the error diffusing process, the multivalue pixel data which was inputted is binarized by a predetermined threshold value and errors between the pixel data and the threshold value are two-dimensionally diffused in accordance with weight coefficients of an error diffusion matrix. Thus, a density as a whole image can be preserved and a half-tone image can be falsely obtained. By two-dimensionally diffusing the errors, a smooth half-tone image which is visually beautiful can be obtained.
Reference numeral 2006 denotes a data control unit to control a transfer of image data in accordance with each of the transmission mode, reception mode, and copy mode; 2007 a resolution conversion unit to convert a resolution of the image data to a resolution of a printer when the image data is printed and recorded; 2008 a density correction unit to perform a density correction to the image data which was resolution converted by the resolution conversion unit 2007; and 2009 an ink jet printer to print and record. In the conventional apparatus, a printer of a resolution of (360 dpi.times.360 dpi) is used.
Reference numeral 2010 denotes a modem to modulate and demodulate the image data upon transmission or reception; 2011 an NCU to control an office line upon transmission or reception; and 2012 a CPU to control each of the above units.
The operation of the conventional apparatus will now be described hereinbelow.
In the transmission mode, a transmission original is read by the CS 2001 at a resolution of 8 dots/mm in the main scanning direction and, for example, 7.7 lines/mm in the sub scanning direction by the selection of the operator. A read output of the original is corrected by the analog image processing unit 2002 and is converted to multivalue digital data every pixel by the A/D conversion unit 2003.
The digital data is LOG converted to the density data by the luminance/density conversion table unit 2004 and is binarized by the error diffusion processing unit 2005, so that the binary image data is outputted. In the conventional apparatus, the image data which is obtained here has a resolution of 8 dots/mm in the main scanning direction and 7.7 lines/mm in the sub scanning direction.
The image data is sent to the modem 2010 by the data control unit 2006 and is modulated to the analog transmission signal and is transmitted to an office line through the NCU 2011.
In the reception mode, the analog transmission signal which arrives from the office line is sent to the modem 2010 through the NCU 2011 and is demodulated to the digital image data. However, a resolution in this instance is set to 8 dots/mm in the main scanning direction based on the G3 standard. It is now assumed that the data of a resolution of, for example, 7.7 lines/mm in the sub scanning direction was received.
The data control unit 2006 transfers the image data to the resolution conversion unit 2007 and its resolution is converted.
Namely, to record the data of (8 dots/mm.times.7.7 lines/mm) by the ink jet printer 2009 of (360 dpi.times.360 dpi), pixels are added in accordance with a predetermined pattern of the image data so as to increase the number of pixels in the main scanning direction by about 1.77 times and in the sub scanning direction by about 1.84 times.
In the ink jet system, in case of seeing the whole image, the recording image becomes too dense than the actual original image because of a cause such as a blur of the ink or the like.
Therefore, the density correction unit 2008 executes a density correction by replacing the black pixels to the white pixels in accordance with the pattern of the image data in order to avoid a phenomenon such that the image density when the image is printed by the ink jet printer 2009 seems to be dense due to an increase in number of pixels by the resolution conversion mentioned above.
The data is sent to the ink jet printer 2009 and is recorded as an image at a resolution of (360 dpi.times.360 dpi).
In the copy mode, the original image read by the CS 2001 is processed in a manner similar to the case in the transmission mode and is obtained from the error diffusion processing unit 2005 at a resolution of, for example, (8 dots/mm.times.7.7 lines/mm) as binarized image data.
The image data is transferred to the resolution conversion unit 2007 by the data control unit 2006.
The image data is, further, resolution converted from a resolution of (8 dots/mm.times.7.7 lines/mm) to a resolution of (360 dpi.times.360 dpi) by the resolution conversion unit 2007 in a manner similar to the reception mode. In a manner similar to the case of the recording of the reception data mentioned above, in order to avoid that the image becomes too dense as a whole, the density correction unit 2008 inverts the image data from the black pixels to the white pixels and, after that, the data is recorded as an image at a resolution of (360 dpi.times.360 dpi) by the ink jet printer 2009.
In the conventional apparatus as mentioned above, however, the data after it was binarized by the error diffusing process or the like is resolution converted. Namely, in the copy mode, the image data read by a resolution of for example, (8 dots/mm.times.7.7 lines/mm) is binarized by the error diffusing process, the pixels are added to resolution convert the image processing pattern obtained to the data of the resolution,(360 dpi.times.360 dpi) of the printer, and the pixels are further replaced in order to correct the density as a whole image for the above data. Therefore, after the original image was read, the image processing pattern (smooth half-tone image) formed by the error diffusing process is broken by those pixel operations, so that the output image is remarkably deteriorated as compared with the original image. The replacing process of the pixels as mentioned above becomes complicated.
Since an algorithm for the luminance/density conversion ordinarily has only one pattern, the density correction according to the resolutions of the input image data and the image data that is outputted cannot be properly executed. When the processing method at the stage before the resolution conversion is executed differs, even in the case where those resolutions are equal, the density as a whole image seems to be different.
The above points cause large problems since a significance of the copy function is increasing in the widespread of the use of the normal paper as a recording paper as mentioned above.